1. Field of the Invention
The present invention relates to a coil type high-voltage resistive cable for preventing noise, in which cable the outer surface of a core is wound with a resistance wire having a specified electrical resistivity, in a direction normal to the longitudinal axis of the core, and then coated with an insulator layer.
2. Description of the Prior Art
A high-voltage generated in an ignition coil is applied by way of a distributor or directly to a spark plug. In view of this, there have been used two types of high-voltage resistive cables for connecting the ignition coil and the spark plug: braid type obtained by twisting fibers impregnated with carbon; and coil type obtained by winding a thin metal wire having a high electrical resistivity around a core of magnetic material, etc. High-voltage resistive cables of both types are required to have a low transmission loss, excellent heat and voltage resistances and to display a good noise preventing effect against noises resulting from spark ignition of an engine.
Wires disclosed in, e.g., Japanese Examined Utility Model Publications Nos. 1-32253 and 6-6418 are known as prior art coil type high voltage resistive cables for preventing noise.
Specifically, the coil type high voltage resistive cable disclosed in the former publication is as follows. A mixture obtained by mixing 300 to 700 parts by weight of ferrite powder with 100 parts by weight of base polymer is extruded to coat a center reinforced braid obtained by twisting aramid fibers, thereby obtaining a ferrite core having an outer diameter of 1.3 mm or smaller. A resistance wire is wound around the outer surface of the ferrite core at a pitch of 8000 to 14000 winds/m in a direction normal to the longitudinal axis of the ferrite core. Polyolefin resin is extruded to coat the outer surface of the ferrite core wound with the resistance wire, thereby forming an insulator layer. Further, a sheath is formed around the outer surface of the insulator layer. As a specific example, this publication discloses: the outer surface of aramid fibers of 1500 denier is coated with the mixture obtained by mixing Mn-Zn ferrite powder with chlorinated polyethylene, and a nichrome (Ni-Cr) wire having a diameter of 0.06 mm and an electrical resistivity of 105 .mu..OMEGA..cm is wound around the outer surface of the ferrite core at a pitch of 9600 winds/m to set the resistance value of the entire resistance wire as a conductor at 16 k.OMEGA./m.
On the other hand, the coil type high-voltage resistive cable disclosed in the latter publication is as follows. Silicon rubber mixed with ferrite powder is extruded to coat a tension member consisting essentially of aramid fibers, thereby forming a core. A stainless wire or like resistance wire having a diameter of 0.055 mm is wound around the outer surface of the core at a pitch of 14000 winds/m. A partially conductive resin layer having a uniform thickness of 4 to 8 .mu.m and an electrical resistivity of 10.sup.2 to 10.sup.5 .OMEGA..cm is formed on the core wound with the resistance wire, for example, by dipping this core in molten epoxy resin mixed with carbon.
There has been developed a so-called lean-burn engine for burning lean fuel mixture in view of an exhaust gas control for automotive vehicles which will be enforced as a countermeasure to the environmental problems in recent years. Such an engine is required to have a higher ignition energy than normal engines, thus making it necessary to reduce a resistance value of the aforementioned coil type high-voltage resistive cable used to connect the ignition coil and the spark plug, for example, to 1/2 of that of the prior art resistive cable.
However, in the case of the above coil type high-voltage resistive cable for preventing noise, if the winding pitch of the resistance wire is reduced in order to lower the resistance value of the resistive cable, the inductance of the resistive cable becomes smaller, resulting in a reduced noise preventing effect. In order to avoid this, the resistance value of the resistive cable may be reduced by using a thicker resistance wire without reducing the winding pitch. In this case, the short-circuiting of the densely wound resistance wire may cause an abnormal reduction in the resistance value and a reduction in the noise preventing performance.
The short-circuiting of the resistance wire normally occurs when a spacing between adjacent winds of the resistance wire is smaller than the diameter of the resistance wire. In order to prevent this short-circuiting, it may be considered to form a partially conductive resin layer on the resistance wire as disclosed in the above publication (Japanese Examined Utility Model Publication No. 6-6418). However, this leads to a higher manufacturing cost and is thus economically disadvantageous.
In view of the above problem, it is an object of the invention to provide a coil type high-voltage resistive cable for preventing noise, which cable has a lower resistance value than and a noise preventing performance substantially similar to a prior art resistive cable while maintaining an inductance level without increasing the diameter of the resistance wire and reducing the winding pitch thereof.